1. Field of the Invention
The invention relates to a device for the extrusion of multi-layer plastic films, boards or tubes with a feed block, whose inputs are fed with polymer melts and which conveys the polymer melts from its inputs via individual layer channels to a slotted passage, forming its output.
2. Description of the Related Art
To manufacture flat films, there exist extruders with slit dies, from which various polymers are extruded into a sandwich-like arrangement of layers. In the case of this kind of slit die, which is disclosed, for example, in the DE 39 29 002A, different melt streams, issuing from several extruders, are conveyed through a feed block into a sandwich-like arrangement of layers, after which process the sandwich-like melt spreads out in a clothes hanger-like cavity while retaining its sandwich-like arrangement of layers, so that an expanded polymer strand is extruded, in which the sandwich-like arrangement of layers of different melts is retained.
The DE 92 16 920 U discloses an extrusion die, which utilizes and modifies in such a manner the conversion of different polymer melts, which issue, as explained above, from the slit dies, into a sandwich-like layered strand and the final expansion of this strand in a clothes-hanger-like cavity while retaining the sandwich-like layer that the sandwich-like cavity is curved in the shape of a circle so that its legs converge into each other and the cavity changes into the subsequent annular channel, which leads to the annular gap of the extrusion die.
If at this point two or more polymer melts with different viscosity are processed in one of the prior art extrusion dies, the layer thickness is automatically unevenly distributed over the width of the coextruded films. The polymer melt with the lower viscosity expands more in the edge region than in the center region. To avoid these uneven layer thicknesses caused by the prior art feed blocks, mechanical control units, such as pins or restrictor bars, were used that were supposed to have such an impact on the individual layers of polymer melts that, after issuing from the die, a uniform individual layer profile is obtained. Since these mechanical systems usually dip into the melt stream, in order to have an effect, they leave behind them a so called dead water region, i.e., a space where the melt flows more slowly and thus tends to burn.
Therefore, the object of the present invention is to provide a device to extrude multilayer plastic films, boards or tubes, in which process the individually coextruded layers exhibit the same layer thickness even when the polymer melts that are used exhibit different viscosity.
The invention solves this problem with the aforementioned class of device for extruding multilayer plastic films, boards or tubes in that temperature control units are arranged in the feed block parallel to the individual layer channels for the purpose of heating or cooling. Thus the temperature of the respective polymer melt stream and thus its viscosity can be set by means of several temperature control units, arranged side-by-side, over the width of the individual layer channels. In this manner the flow characteristics of the polymer melt stream can be adjusted to the viscosity and thus the flow characteristics of the coextruded polymer melt streams. The results are uniform layer thicknesses without the need for mechanical control units.
It is especially advantageous if the many temperature control units, arranged side-by-side, are thermally separated from each other. One example thereof constitutes separating walls, made of a heat insulating material for thermal separation. Thus it is guaranteed that the respective temperature control elements affect only the area of the polymer melt stream, whose viscosity is supposed to be affected in a specific way.
To produce a multilayer plastic film or board, a clothes hanger-like expansion of the melt line, as disclosed in principle in the DE 39 29 002 A1, can be attached to the feed block.
As an alternative a multilayer plastic tube can be formed by attaching a connecting adapter, which empties into an annular distributing chamber, which, when active, is designed approximately like a clothes hanger, and whose apex is centered relative to the connecting adapter, and whose legs wind around the internal insert, bordering the annulus, so as to increasingly approach the die gap, and which changes into an annular channel, leading to the die gap. This arrangement is already known from the DE 92 16 920 U of the applicant.